This invention relates to a method and apparatus for minimizing the effects of temperature on integrated circuit performance. More specifically, the invention provides a means for minimizing the effects of temperature on the electrical performance of field effect transistor NET) integrated circuits (ICs).
Field effect transistors (FETs) and more specifically high electron mobility transistor (HEMT) integrated circuits have widespread application in many areas including aerospace and mobile communication systems. When FET or HEMT integrated circuits are exposed to variations in temperature, their electrical performance characteristics can vary. Many applications for such circuits require consistent operation over a broad range of temperatures. To achieve temperature-invariant operation of FET or HEMT ICs, numerous temperature compensation (TC) solutions have evolved, with varying degrees of success. The existing solutions can be classified into three general approaches.
The first approach involves the use of external temperature sensors and direct-current amplifiers to generate a control signal that is delivered to the integrated circuit. Common candidates for this solution are Microwave Monolithic Integrated Circuits (MMICs). This is perhaps the most obvious temperature compensation method, and has been in use for decades. Embodiments of the idea, as applied to high frequency field-effect transistor (FET) amplifiers, can be found in U.S. Pat. No. 5,373,250 (Gatti et al., 1994) and U.S. Pat. No. 5,724,004 (Reif et al., 1998). The main disadvantage of this method is that external circuits must be added for the temperature compensation function. These external circuits require temperature-variable elements such as thermistors that are not easily integrated on-chip. Additionally, the thermal sensor elements must be located in close proximity to the MMIC in order to accurately provide a meaningful temperature reading. This proximity requirement further complicates the assembly of the temperature-compensated circuit embodying this solution.
The second approach commonly employed utilizes an on-chip temperature sensor, such as a diode, in conjunction with an external feedback control loop. This is a variation of the first approach, where the temperature-sensing element has been integrated into the MMIC chip. The sensor is typically a monolithic diode that is biased by a constant current from an external source to produce a terminal voltage inversely proportional to temperature. This voltage is amplified and used either to control the bias of active devices on the IC or to control a variable attenuator to adjust the response of the circuit as it changes with ambient temperature (c.f., Long, et al., xe2x80x9cWideband HEMT MMIC Low-noise Amplifier with Temperature Compensation,xe2x80x9d Electronics Letters, Mar. 3, 1994, pp. 422-423; also Dow, et al., xe2x80x9cMonolithic Receivers with Integrated Temperature Compensation Function,xe2x80x9d 1991 IEEE GaAs IC Symposium Digest, pp. 267-270). The disadvantages of this method include the size, the cost, and the assembly complications arising from the addition of external control circuits, which typically require individual adjustment to match the temperature characteristics of each MMIC.
The third approach utilizes an on-chip direct-current feedback amplifier to regulate the bias of the active radio frequency (RF) circuits. In this approach, a direct-current amplifier is integrated into the MMIC in order to regulate the bias current applied to the active RF devices (Kobayashi, et al, xe2x80x9cMonolithic Regulated Self-biased HEMT MMICsxe2x80x9d IEEE Trans. Microwave Theory Tech., December 1994, pp. 2610-2616; also U.S. Pat. No. 5,387,880). While this monolithic solution is the most compact scheme in the literature, it has disadvantages when applied to high-performance HEMT MMICs with low direct-current power dissipation, such as are required in satellite communications circuitry. Integrating the active bias regulator on-chip involves designing precise direct-current operational-amplifiers into a semiconductor process optimized for high-frequency RF devices. Such devices are not well suited for these types of precise direct-current operational-amplifiers. This compromise results in an inefficient regulator, which may more than double the power consumption of the IC.
The present invention provides both methods and circuits that are simpler, more robust, more compact and significantly more power-efficient than previous, comparable circuits.
One embodiment of the present invention includes an apparatus comprising a temperature compensating epitaxial resistor having a first resistive component and a second resistive component, wherein the first resistive component includes an electrically isolated semi-conducting channel wherein the channel has a width and a length. The electrical resistance of the semiconductor is functionally dependent on the channel geometry. The second resistive component is comprised of a plurality of ohmic contacts. The ohmic contacts are made of a substantially conducting material such as a heavily doped semiconductor or a transition metal. The ohmic contacts are functionally interfaced with the first resistive component, and the ohmic contacts have electrical resistance that is functionally dependent on temperature and geometry. The first and second resistive components have opposite, and independently adjustable, functional temperature dependences.
Another embodiment of the present invention comprises a temperature compensated field effect transistor wherein the transistor includes, an epitaxial resistor having a first resistive component and a second resistive component, wherein the first resistive component includes an electrically isolated semi-conducting channel wherein said channel has a width and a length. The electrical resistance of the semiconductor is functionally dependent on the channel geometry. The second resistive component is comprised of a plurality of ohmic contacts, wherein the ohmic contacts are functionally interfaced with the first resistive component, and wherein the ohmic contacts have electrical resistance that is functionally dependent on temperature and geometry. The first and second resistive components have opposite, and independently adjustable, functional temperature dependences.
Yet another embodiment of the present invention relates to a method for controlling or substantially eliminating temperature induced performance variations in field effect transistor circuits wherein the method includes integrating a thermally compensating resistor into the field effect transistor circuits, wherein the resistor compensates for temperature induced performance variations by responding to, and compensating for, temperature induced performance variations.
In yet another embodiment the present invention uses an epitaxial resistor to control temperature induced performance variations wherein the epitaxial resistor having a first resistive component and a second resistive component, wherein the first resistive component includes an electrically isolated semi-conducting channel wherein the channel has a width and a length. The electrical resistance of the semiconductor is functionally dependent on the channel geometry. The second resistive component is comprised of a plurality of ohmic contacts, wherein the ohmic contacts are functionally interfaced with the first resistive component, and wherein the ohmic contacts have electrical resistance that is functionally dependent on temperature and geometry. The first and second resistive components have opposite, and independently adjustable, functional temperature dependences.
Yet another embodiment of the present invention includes an apparatus for locally measuring temperature within a circuit, where the apparatus includes at least one reference resistor and at least one temperature-indicating resistor. The reference resistor or resistors have a specified temperature dependence, and the temperature indicating resistor or resistors ideally have a substantially constant temperature functionality. The electrical properties of the different resistors are compared, and provide a temperature signal. One example of such a comparative system would include a Wheatstone bridge, where two temperature independent resistors and two temperature dependant resistors having opposite temperature dependences are used.